Work to move charge to infinity problem

[awertag]

Homework Statement

The figure below shows three charges at the corners of a rectangle of length x = 0.55 m and height y = 0.35 m.

http://www.webassign.net/walker/20-23alt.gif

(a) How much work must be done to move the +2.7-µC charge to infinity?

Homework Equations

W=(\DeltaV)(q)
V3=potential at the spot of the charge
Q3=2.7e-6 C

The Attempt at a Solution

W=\DeltaVq
W=V3(Q3)
then try to solve for V3=KQ3/r3 but i know i can't because i don't have an r because there's no reference point for it. So, is there another way to solve for potential or am i just going about the whole thing wrong?

thanks very much

--aweg

 

[awertag]

i don't know why it looks like that sorry, i can't seem to fix it :(

 

[Piyu]

Electric potential is a scalar. Just do it seperately for the charge in question and each individual other charge and then sum them up(superposition principle).

Finding the starting distance should be easy with some trigo. Then simple integrate QdV.

 

[awertag]

right my only problem is i don't having a starting place. unless i assume it's the corner with no charge?

 

[awertag]

if the format bothers you, I reposted it here: https://www.physicsforums.com/showthread.php?p=3159368#post3159368

 

[Piyu]

Hmm, what do you mean no starting place? You're moving the 2.7 uC charge to infinity, that distance to each of the other charge would be the starting distance.

 

[awertag]

But that doesn't go towards what I'm trying to find, i think?

 

[Piyu]

Ah sorry, wasn't reading properly.

Work done is charge * change in potential(Vfinal - Vinitial).

V at final is set to be zero.

(In fact we can just simply use the electric potential energy formula and subtract).

Do the work done seperately between the 2.7 charge and the 2 charges and add them up.

 

[awertag]

Right that's what I had above: W=(V3)(Q3). But I don't understand how to get V3.

 

[awertag]

Why work done between the charge and each of the other two?

 

[Piyu]

V3, just apply the formula for potential between 2point charges, u got length x and height y. u can figure out the r using those.

Sorry, thought u had trouble finding the work done. Erm i was going to use superposition of the 2 work done by working them out seperately. Alternatively, u could also just sum up the 2 potentials of the 2 negative charges and label it V3.

 

[awertag]

Ah, that's what i was looking for! got it with the v2+v1=v3 so work was .3925 J.
Thanks for all the help on both questions!

 

[Piyu]

NP glad to be of help :) Have a nice day! :P